Crystal structure, spectroscopic characterization and DFT study of two new linear fused-ring chalcones

A structural comparative study between two chalcones was undertaken and some effects on geometrical parameters, such as planarity and dihedral angles, are described.

The structures of two new anthracenyl chalcones, namely (E)-1-(anthracen-9yl)-3-(4-nitrophenyl)prop-2-en-1-one, C 23 H 15 NO 3 , and (E)-1-(anthracen-9-yl)-3-(4-iodophenyl)prop-2-en-1-one, C 23 H 15 IO are reported. A structural comparative study between the two chalcones was performed and some effects on the geometrical parameters, such as planarity and dihedral angles, are described. The molecular geometry was determined by single-crystal X-ray diffraction, and density functional theory (DFT) at B3LYP with the 6-311++G(d,p) basis set was applied to optimize the ground-state geometry. In addition, intermolecular interactions responsible for the crystal packing were analysed. The electronic properties, such as excitation energies and HOMO-LUMO energies were calculated by time-dependent density functional theory (TD-DFT) and the results complement the experimental findings. The molecular electrostatic potential (MEP) was also investigated at the same level of theory in order to identify and quantify the possible reactive sites.

Chemical context
The synthesis of new organic molecules and the characterization of their molecular properties are the necessary prerequisites for further research in modern technologies. Conjugated organic chalcone molecules are recognized to be promising materials in the field of opto-electronic applications (Aggarwal et al., 2001). The materials are characterized by an extremely excited -conjugated chain with strong electron acceptor-donor pairs at the end (D--A) of the terminal rings (Manjunath et al., 2011). Chalcone derivatives are an interesting type of organic NLO materials that can be tuned to match particular requirements. In these systems, two aromatic rings have to be substituted with suitable electron-donor or acceptor groups to increase the asymmetric charge distribution in either or both the ground state and excited states, giving rise to an enhanced optical non-linearity (Rajesh Kumar et al., 2012). Meanwhile, the enone moiety acts as the -conjugated bridge that is responsible for intermolecular charge transfer between the donor and acceptor substituent groups. The title compounds contain an anthracene fused-ring system (strong electron donor) containing a nitro group or an iodine atom (strong electron acceptor) substituted at the para terminal position. Their investigation included characterization using UV-vis spectroscopy and computed studies of HOMO-LUMO energy gaps and molecular electrostatic potential (MEP).

Structural commentary
The molecular structures of the compounds (I) and (II) are shown in Fig. 1a. All geometrical parameters are within normal ranges and comparable with those in the previously reported structure of anthracenyl chalcones (Zainuri et al., 2018a). The optimization of the molecular geometries ( Fig. 1b) leading to energy minima was achieved using DFT [with Becke's non-local three parameter exchange and the Lee-Yang-Parr correlation function (B3LYP)] with the 6-311++G (d,p) basis set as implemented in Gaussian09 program package (Frisch et al., 2009).

Table 2
Hydrogen-bond geometry (Å , ) for (II). 4. UV-Vis absorption analysis and frontier molecular orbital (FMO) energies TD-DFT calculations at the B3LYP/6-311G++(d,p) level were performed to simulate the absorption characteristics and obtain information about the excited states. The experimental spectrum (Fig. 4) shows peaks at wavelengths of 318, 366 and 386 nm in (I) and 321, 367 and 387 nm in (II) with the wavelength of maximum absorbance being observed at 386 nm in (I) and 387 nm in (II). The absorption maxima are assigned to the -* transitions, i.e. the transition of an electron from a bonding () to an anti-bonding ( * ) molecular orbital, which are attributed to the C O groups and aromatic ring excitations. The experimentally measured spectra of both compounds match those of the simulated chalcones, which have maxima at 395 nm for (I) and 394 nm for (II). The difference in energy of the HOMO and LUMO is an important index that provides information about the chemical stability of molecules since these energies are directly related to the ability to donate and accept electrons. In the ground state (HOMO), the charge densities are mainly delocalized over the anthracene ring systems and the enone moiety, while in the LUMO state, the charge densities are accumulated on the nitrobenzene ring and the enone moiety in (I), and the iodobenzene ring in (II). A small HOMO-LUMO gap automatically means small excitation energies to the manifold excited states and a large HOMO-LUMO gap implies high stability with respect to chemical reactions (Custodio et al., 2017). The HOMO-LUMO energy gaps (Fig. 5) are computed to be 2.93 eV and 2.81 eV, respectively, for (I) and (II). In the experimental results, the value of energy gap was estimated from the absorption curve by extrapolating the linear portion of the curve to zero absorption, giving values of 3.14 eV for (I) and 3.07 eV for (II). These values for the band gaps suggest that the materials are dielectric in nature (Suguna et al., 2015), dielectric materials having wide transparency in the UV region. Such materials with wide transparency are required for the fabrication of optical electronic devices.

Molecular electrostatic potential (MEP)
The importance of the MEP lies in the fact that it simultaneously displays molecular size and shape as well as positive, negative and neutral electrostatic potential regions in terms of colour grading and is useful in investigating relationships between molecular structure and physicochemical properties (Murray & Sen, 1996;Scrocco & Tomasi, 1978). The MEP maps for the molecules of (I) and (II) were calculated theoretically at the B3LYP/6-311G++(d,p) level of theory and the UV-Vis absorption spectra for compounds (I) and (II).

Figure 5
The electron distribution of the HOMO and LUMO energy levels in compounds (I) and (II). obtained plots are shown in Fig. 6. The negative red regions are concentrated at the oxygen atoms, showing the electrophilic sites. Hence, the oxygen atoms are the most reactive sites for nucleophilic attack, as well as the more proper sites to attack the positive regions of the receptor molecule. The negative potential values of compounds (I) and (II) are À0.049 a.u and À0.649 a.u., respectively. The blue regions indicate areas of positive charge concentration, which are concentrated over the hydrogen atoms and iodine substituent atom, indicating the nucleophilic sites. Green regions represent areas with zero potential.

Synthesis and crystallization
9-Acetylanthrancene (0.5 mmol) was dissolved in methanol (20 ml) for about 10-15 mins. Then 4-nitrobenzaldehyde (0.5 mmol) [for (I)] or 4-iodobenzaldehye (0.5 mmol) [for (II)] was added and the solution was stirred for another 10-15 min. Then, NaOH was added and after stirring for 5 h, the reaction mixture was poured into cold water (50 ml) and stirred for 5-10 min. The precipitated solid was filtered, dried and recrystallized from acetone solution to obtain the corresponding chalcones.

(E)-1-(Anthracen-9-yl)-3-(4-nitrophenyl)prop-2-en-1-one (I)
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.